The present invention relates to rotary regenerative air preheaters for the transfer of heat from a flue gas stream to an incoming combustion air stream and particularly to the configuration of the heat transfer elements for the air preheater and the method of manufacturing those elements.
A rotary regenerative heat exchanger is employed to transfer heat from one hot gas stream, such as a hot flue gas stream, to another cold gas stream, such as combustion air. The rotor contains a mass of heat absorbent material which first rotates through a passageway for the hot gas stream where heat is absorbed by the heat absorbent material. As the rotor continues to turn, the heated absorbent material enters the passageway for the cold gas stream where the heat is transferred from the absorbent material to the cold gas stream.
In a typical rotary heat exchanger, such as a rotary regenerative air preheater, the cylindrical rotor is disposed on a horizontal or vertical central rotor post and divided into a plurality of sector-shaped compartments by a plurality of radial partitions, referred to as diaphragms, extending from the rotor post to the outer peripheral shell of the rotor. These sector-shaped compartments are loaded with modular heat exchange baskets which contain the mass of heat absorbent material commonly formed of stacked plate-like heat transfer elements.
Conventional heat transfer elements for regenerative air preheaters are form-pressed or roll-pressed steel sheets or plates which are then stacked to form the mass of heat transfer material. One typical arrangement is for the plates to be formed with spaced apart ridges, usually double ridges projecting from opposite sides of the plate, which extend along the plate either in the direction of flow or obliquely thereto and which serve to space the plates from each other. The spacing forms the flow channels between the plates for the flow of flue gas and air. For examples of such heat transfer elements, reference is made to U.S. Pat. Nos. 4,744,410 and 4,553,458.
One of the effects of using these ridges to provide the spacing of the heat transfer elements is that they form flow paths through the bundle of heat transfer elements which are larger in cross-sectional area per surface area of exposed plate surface than the cross-sectional area per surface area of the other portions of the plate. This results in lower flow resistance, less turbulence and mixing, greater mass flow of gas and air and lower heat transfer as compared to the remainder of the plates. Therefore, although the ridges do provide structural integrity and accurate spacings, they have their negative effect on heat transfer.